1. Field of the Invention
The present invention relates to a wire bonding apparatus with a bonding arm supported so as to move upward and downward by an elastic member and also to a method for controlling the apparatus
2. Prior Art
A wire bonding apparatus in which the bonding arm is supported so as to be movable upward and downward and driven by an elastic member is shown in FIG. 4.
The bonding arm 2 of this bonding apparatus has a bonding tool 1 at one end thereof and is fastened to one end of a supporting frame 3. The supporting frame 3 is attached to a moving table 5 via a plate spring 4 which is assembled in the form of a cross, so that the supporting frame 3 is swingable upward and downward as shown by an arrow V, and a moving table 5 is mounted on an XY table 6. The coil 8 of a linear motor 7 is fastened to another end of the supporting frame 3, and the magnet 9 of this linear motor 7 is fastened to the moving table 5. A linear scale 10 is attached to the rear end (right-side end in FIG. 4) of the supporting frame 3.
Examples of a wire bonding apparatus of this type are described in Japanese Patent Application Pre-Examination Publication (Kokai) Nos. S58-184734 and H6-29343 and Japanese Examined Patent Application Publication (Kokoku) No. H6-80697.
With the structure described above, the supporting frame 3 and bonding arm 2 are caused to swing in the direction of arrow V about the cross-shaped plate spring 4 by the linear motor 7, and the bonding tool 1 is, as a result, moved up and down. Furthermore, the moving table 5, supporting frame 3, bonding arm 2 and bonding tool 1 are moved horizontally (or in X and Y directions) by the XY table 6. By way of the combination of the vertical movement and horizontal movement of the bonding tool 1 as described above, a wire 12 passing through the bonding tool 1 is connected between the first and second bonding points on the workpiece (not shown). In other words, a ball 13 formed at the tip end of the wire 12 is bonded to the first bonding point, and then the other portion of the wire 12 is bonded to the second bonding point During this bonding of the wire 12 to the first and second bonding points, a load or a bonding load is applied by the linear motor 7 so that the ball 13 and wire 12 is pressed against the bonding points on the workpiece by the bonding tool 1.
Next, the operation system for the above bonding apparatus and the control configuration of the linear motor 7 will be described.
The operation system substantially comprises an external input-output means 20 and a computer 21. The external input-output means 20 is used for inputting and outputting various types of information (required for the operation of the apparatus) with respect to the computer 21. The computer 21 comprises a control circuit 22, an operating circuit 23, a reference coordinate register 24 and a height position counter 26. The control circuit 22 controls the external input-output means 20, operating circuit 23, reference coordinate register 24 and height position counter 26.
In the reference coordinate register 24, the height position of the bonding arm 2 is stored. More specifically, the value of the height position is inputted into a position control circuit 30 as one position command. When the value is thus inputted, the position control circuit 30 compares a previous position command and a new position command and generates an amount of movement of the bonding tool based upon the difference between the two position commands. This amount of movement is transmitted to a motor driver 31 as a driving signal 33.
The motor driver 31 generates electric power which is used to move the bonding tool 1 to a designated height position in accordance with the driving signal 33. Generally, electric power is the product of voltage and current; therefore, actual control of the linear motor 7 can be accomplished by controlling either the voltage or current, or both. Accordingly, the following explanation describes the case where the driving current 35 (and not a driving voltage) that flows through the linear motor 7 is controlled. The circuit described in Japanese Examined Patent Application Publication (Kokoku) No. H6-18222 may be cited as an example of the circuit that controls the driving current. When the driving current 35 generated by the motor driver 31 is applied to the coil 8 of the linear motor 7, a driving force is generated; and as a result of this driving force, the supporting frame 3, bonding arm 2 and bonding tool 1 are caused to swing about the plate spring 4 (or moved up and down).
Furthermore, the height position counter 26 of the computer 21 counts signals from an encoder 32 which converts signals from the position sensor 11 into a signal format which can be inputted into the computer 21 and generates an actual height position of the linear scale 10. The computer 21 is provided beforehand with a ratio of the amount of movement of the bonding tool 1 in the vertical direction to the amount of movement of the linear scale 10 in the vertical direction, and a quantization coefficient of the position sensor 11, i. e., a coefficient which converts the amount of movement into an electrical signal. Accordingly, the actual height position of the bonding tool 1 is determined by calculating the value indicated by the height position counter 26 via the operating circuit 23 based upon the value described above. The term "height position of the bonding tool" refers to the height position at which the bonding tool 1 contacts the object to which a load is to be applied.
The bonding arm 2 and bonding tool 1 swing about a fulcrum 4a of the cross-shaped plate spring 4. Accordingly, it is desirable that the bonding tool 1 be in a vertical position; in other words, it is desirable that the bonding arm 2 be in a horizontal position when the bonding tool 1 contacts the bonding point. With the bonding arm 2 thus adjusted to a horizontal position, an instruction to place the bonding arm 2 in a horizontal position is sent to the computer 21 by the external input-output means 20. As a result of this instruction, the control circuit 22 sends control information for this purpose to the position control circuit 30 via the reference coordinate register 24; and from the position control circuit 30, a driving signal 33 which produces the driving current 35 is sent to the motor driver 31. On the basis of this driving signal 33, the motor driver 31 produces the driving current 35 of a specified polarity and magnitude and outputs this driving current 35 to the coil 8.
Instructions concerning the movement of the bonding arm 2 are thus transmitted from the computer 21 in this manner.
In the system wherein the supporting frame 3 is supported by the plate spring 4 so as to swing upward and downward as described above, when no driving current 35 flows through the coil 8 or when no driving force is generated in the linear motor 7, the bonding arm 2 stops at the equilibrium position B as shown in FIG. 5. The equilibrium position B is a balanced position where the driving force of the plate spring 4 and the weight balance of the bonding tool 1, bonding arm 2, supporting frame 3, coil 8 and linear scale 10, etc. are supported by the plate spring 4.
In other words, the driving force of the plate spring 4 in this case acts in a direction which causes the bonding arm 2 to return to the equilibrium position B. More specifically, when the bonding arm 2 is in a position A which is higher than the equilibrium position B, the plate spring 4 generates a driving force which pushes the bonding arm 2 downward toward the equilibrium position B; and when the bonding arm 2 is in a position C or D which is lower than the equilibrium position B, then the plate spring 4 generates a driving force which pushes the bonding arm upward toward the equilibrium position B.
The equilibrium position B varies according to the mechanical factors of the wire bonding apparatus; therefore, the bonding arm 2 in the equilibrium position B is not necessarily in a horizontal position. Here, the term "mechanical factors" refers to mechanical errors arising from the working precision, assembly, adjustment, etc. of the parts of the bonding head.
In cases where the output is caused to follow the command speed, etc. in ordinary motor speed control, a constant deviation (i. e., a deviation between the command value and the output which is generated after the output has reached a steady state with respect to elapsed time after passing through a transitional state) is always generated; accordingly, the control circuit is endowed with integrating characteristics (I-control) so that the constant deviation is controlled to zero. In other words, since the deviation is integrated, a cumulative output appears until the deviation reaches zero. However, the installation of an integrating means within the system results in the generation of a delay which affects the stability of the control system; accordingly, in the control shown in FIG. 4, since the stability of the control system is important, control with an integrating means installed in the control circuit is not performed. Consequently, because of external perturbation, the target height position of the bonding arm cannot be maintained, and deviations (positional deviations) occur. Incidentally, the term "I-control" refers to a control in which the value obtained by integrating the deviation between the command value and the fed-back output is taken as the amount of movement of the control system.
As the bonding arm 2 is displaced toward the upper position A which is higher than the balance position B, the driving force of the plate spring 4 which brings down the bonding arm 2 increases; on the other hand, as the bonding arm 2 is displaced toward the lower positions C and D which are lower than the balance position B, the driving force of the plate spring 4 which brings up the bonding arm 2 toward the balance position B increases. If a positional correction feedback which is sufficient to correct for the difference in the spring force of the plate spring 4 according to the positions A, C and D is performed, i. e., if the position feedback gain level is increased, the bonding arm 2 begins to vibrate above a certain level. Accordingly, the position feedback gain is lowered to a level at which no vibration occurs. However, if the position feedback gain level is lowered, it becomes impossible to completely correct the driving force of the plate spring 4 according to the height position, resulting in that a positional deviation occurs.
Recognition of the height position of the bonding tool 1 is accomplished by: detecting the amount of movement of the linear scale 10 in the vertical direction using the position sensor 11; counting the pulse signals 36, which is shown in FIG. 6(a) and outputted by the encoder 32, via the height position counter 26, and then processing the result of this count by the operating circuit 23 through the control circuit 22. A, B, C and D in FIG. 6 indicate the height positions of the bonding arm 2 shown in FIG. 5.
However, as described above, the driving force of the plate spring 4 causes the bonding arm 2 to stop in the positions indicated by inverted triangles in FIG. 6(b), (c) and (d), causing deviations in the stopping positions. Position A+1 indicates a position which is one pitch above position A, and position A-1 indicates a position which is one pitch below position A. Positions B+1, B-1, C+1 and C-1 have similar meanings. Furthermore, in regard to the stopping pitch (with the stopping pitch P (B) at position B used as a standard), the length of one pitch decreases as the stopping position moves upward or downward, because the driving force of the plate spring 4 also increases. Moreover, this variation in the length of one pitch is cumulative, and therefore, the amount of downward deviation increases as the stopping position moves upward, and the amount of upward deviation increases as the stopping position moves downward.
More specifically, the pitch between stops is as shown by Numerical Formulae 1 and 2 below. Furthermore, at position A, as shown in FIG. 6(b), the driving force of the plate spring 4 acts in a direction which causes the bonding arm 2 to drop, so that the stopping position is shifted downward. At position B, as shown in FIG. 6(c), the bonding arm 2 stops at a center point 37 between pulses, since this position is the balanced position. Meanwhile, at position C, as shown in FIG. 6(d), the driving force of the plate spring 4 acts in a direction which causes the bonding arm 2 to rise, so that the stopping position is shifted upward.
[Numerical Formula 1]
P(B)&gt;P(B+1)&gt; . . . P(A-1)&gt;P(A)&gt;P(A+1)&gt; . . . PA1 P(B)&gt;P(B-1)&gt; . . . P(C+1)&gt;P(C)&gt;P(C-1)&gt; . . .
[Numerical Formula 2]
Ordinarily, the stopping control of the height position of the bonding arm 2 or the bonding tool 1 causes the bonding arm 2 to stop at the center 37 between pulses of the pulse signal 36 outputted by the encoder 32. However, as described above, the driving force of the plate spring 4 causes the bonding arm 2 to stop in the positions indicated by the inverted triangles L, so that a shift in the stopping position occurs. Meanwhile, the precision with which the height position of the bonding tool 1 is recognized is determined by how precisely counting can be performed when the signal from the position sensor 11 is converted by the encoder 32 and counted by the height position counter 26; i. e., the precision of recognition is determined by the resolution; and this resolution varies according to the parts that make up the wire bonding apparatus and the circuit construction, etc. Accordingly, even if the position recognized by the computer 21 is accurate, a positional deviation between pulses may be generated depending on the height position.